Oxy-combustion allows an energy saving at least because the energy from the combustion gases is in part not absorbed by the nitrogen in the air. In traditional furnaces, even if a portion of the energy carried with the nitrogen is recovered in the regenerators, the fumes ultimately discharged still carry a significant portion thereof. The presence of nitrogen contributes to this loss.
Moreover, the reduction of energy consumption per production unit in question has the advantage of thus restricting the carbon dioxide emissions, which can reach 25%, and therefore meeting statutory requirements in this field.
The presence of nitrogen is also a source of the formation of so-called NOx oxides, the emission of which is forbidden in practice because of damage associated with the presence of these compounds in the atmosphere. The users have endeavoured to operate the furnaces in conditions that limit emissions as far as possible. In the case of glassmaking furnaces, these practices are not sufficient to meet the very strict standards in force, and it is necessary to undertake costly measures to control pollution from emissions by using catalysts.
The use of oxygen enables the problems associated with nitrogen from the air to be excluded, if not fully, then at least to a very large proportion in the order of 90%.
Despite the advantages outlined above, the use of oxy-combustion in large glassmaking furnaces is yet to be developed. The reasons for this are varied. Firstly, the use of oxygen is necessarily more costly than that of air. Moreover, for the economic balance of using oxy-combustion to be positive requires that different technical problems are overcome, and in particular that of the thermal fluxes comprising the recovery of a significant portion of the heat of the fumes.
The technique described in the unpublished patent application PCT/EP2009/053500 enables the energy yield to be optimised overall by proposing a method for recovering the heat from the fumes to reheat the oxygen used. This optimisation nevertheless involves the use of specially adapted devices, in particular to satisfactorily resist corrosion that is associated with hot oxygen.
The energy recovered to heat the oxygen does not use up that carried by the fumes.
In practice, the energy from the fumes is more than that required to bring the different supplies of the furnace to suitable temperatures for operation. It is firstly a matter of heating the fuel. It is also a matter of preheating the raw materials. With all these elements the conditions of exchange, which take into account the nature of the constituents or the manner in which they can be used, limits the amount of energy that can be recovered for the actual operation of the furnace.
The fuel, whether liquid or gaseous, cannot withstand excessive temperatures. The temperature of the oxygen is limited by that which the devices it circulates in can withstand. In practice, the resistance of the alloys of the conduits used will not go beyond 900-950° C. when operation over long periods must be assured. Also, the raw materials cannot withstand too significant an increase in their temperature without the risk of causing agglomerations that makes working with them more complicated.
A solution to improve the economic balance is to incorporate the production of oxygen at the site of use. This means no specialised companies in its production need to be purchased, and also removes the requirement for transport means or storage arrangements or both.
However, the production of oxygen raises other questions. The usual techniques for the production of oxygen by liquefaction and distillation from air, of the type used by large producers of industrial gases, require extensive installations that can only be profitable with very large production volumes. The quantities necessary for the supply of glassmaking furnaces, even if significant, are usually insufficient to justify such an investment.
Moreover, techniques for oxygen production are known for applications that only require limited quantities of this gas. Among these techniques it has been proposed to use the separation of oxygen from a gaseous mixture, and in particular from air, by means of membranes formed from a selection of materials, also referred to by the term solid electrolytes, that are able to react with oxygen to ionise it on a face of the membrane, to transport these ions through this and reconstitute the gaseous oxygen on the other face of the membrane. Materials of this type are described, for example, in U.S. Pat. No. 5,240,480 or in patent application WO 2008/024405.
To obtain significant yields, the separation of oxygen via a membrane has to proceed at elevated temperature and also a significant difference in the oxygen partial pressure must be maintained on either side of the membrane. For these reasons, the economy of implementing these techniques depends greatly on the precise energy balance of the operation of the installations in question.